1. Field of the Invention
The present invention relates to a sensor having a sensor element for detecting a measurement object, such as a gas sensor for detecting the concentration of a certain component in exhaust gas or a like gas; for example, an oxygen sensor, a NOx sensor, or a HC sensor, or such as a temperature sensor for detecting the temperature of a gas.
2. Description of the Related Art
Conventional sensors employ various kinds of sensor elements (hereinafter, also referred to as elements), such as gas sensor elements, or temperature-sensitive elements, the sensor elements assuming a longitudinally elongated plate-like or rod-like shape. In such a sensor (for example, an oxygen sensor), a sensor element is fixed in the interior of a tubular metallic shell (metallic shell body) having a male-threaded portion to be mounted, for example, to the exhaust gas pipe of an engine. A front-end detection portion thereof projects from the front end of the metallic shell, whereas a rear-end portion thereof projects from the rear end of the metallic shell. In the sensor, usually, a plurality of electrode terminals (terminal electrodes) including electrode terminals used to output a detection output and electrode terminals used to power a heater formed on the element are disposed on opposite side surfaces of a rear end portion of the element while being spaced at electrically insulative intervals in the widthwise direction of the element (laterally). In some cases, in order to establish electrical connection between the electrode terminals and corresponding lead wires extending to the exterior of the sensor, metallic terminal members (metallic terminals or lead frames) connected to corresponding distal ends of the lead wires are pressed against the corresponding electrode terminals by use of their own spring properties so as to abut or be brought into press contact with the electrode terminals (Patent Documents 1 and 2). The metallic terminal members are connected at first ends to corresponding distal ends of the lead wires and are, for example, bent at second ends in a turnback fashion (e.g., in a U-shaped fashion) to thereby form respective plate springs. One leg of each of the U-shaped portions serves as a connection portion (hereinafter, also referred to as an electrode connection portion) which is pressed against the corresponding electrode terminal.
In such a terminal connection structure, as shown in FIG. 12A, metallic terminal members 51 connected to corresponding distal ends of, for example, four lead wires (not shown) are disposed in the interior of a metallic-terminal-member retainer (also called a separator) 71 formed of an electrically insulative material, in a mutually facing fashion while being separated from each other in an electrically insulative condition. As shown in FIG. 12B, a sensor element 21 is inserted, from its rear end 27 through relative movement, between the mutually facing metallic terminal members 51 along its longitudinal direction. By this procedure, the metallic terminal members 51 are elastically deformed outward and are pressed against corresponding electrode terminals 25 formed on side surfaces of the element 21. In other words, insertion of the element 21 forcibly expands outward the space between the facing metallic terminal members 51 against a spring effect of the metallic terminal members 51, and the facing metallic terminal members 51 pinch therebetween the corresponding electrode terminals 25 formed on the side surfaces of the element 21, thereby establishing electrical connection between the electrode terminals 25 and the corresponding metallic terminal members 51. In such a terminal connection structure, for structural reasons, the metallic terminal members 51 are disposed within the metallic-terminal-member retainer 71 such that before connection (before insertion of the element 21), the metallic terminal members 51 are arranged such that a gap (the width of a space) K between the facing metallic terminal members 51 is smaller than the thickness of the sensor element 21 or zero.
In such a terminal connection structure, in order to enhance the reliability of electrical connection, the metallic terminal members 51 must have an intensive spring quality so as to press against the corresponding electrode terminals 25 with a strong spring force in a connected condition. However, intensifying the spring quality of the metallic terminal members 51 increases a force (push-in force) required for inserting the element 21 and involves the risk of undesirable deformation of the metallic terminal members 51 at the time of insertion of the element 21, with resultant deterioration in connection reliability. The reason for this is as follows. The metallic terminal members 51 are formed such that reception portions of the facing metallic terminal members 51 for receiving the element 21 to be inserted fan out toward the element 21 which is to be inserted (fan out downward in the drawing), so as to function as reception guides. Accordingly, at the beginning of insertion of the element 21, edges C of a rear end 27 of the element 21 acutely contact the surfaces of the reception portions of the metallic terminal members 51. For example, in the case where the gap K between the facing metallic terminal members 51 is small, and the metallic terminal members 51 have an intensive spring quality, resistance to insertion (hereinafter, also referred to as insertion resistance) of the element 21 increases, potentially resulting in the metallic terminal members 51 failing to undergo smooth elastic deformation.
For solving the above problem, reducing the insertion resistance of the element is effective. An effective means for reducing insertion resistance is to appropriately chamfer (bevel or round) rear end edges of the element; i.e., edges defined by the rear end surface of the element and the side surfaces of the element on which electrode terminals to be pressed against corresponding metallic terminal members are formed (Patent Document 1). In other words, as shown in FIG. 13, chamfers (chamfered portions) 28 are formed on the rear end 27 of the element 21. Thus, portions of the element 21 which contact the metallic terminal members 51 at the beginning of insertion of the element 21 are not right-angled edges of the rear end 27, but are the chamfers 28, thereby reducing insertion resistance against the metallic terminal members 51 not only at the beginning of insertion but also during the full course of insertion.
Japanese Patent Application Laid-Open (kokai) No. 2001-188060
Japanese Patent application Laid-Open (kokai) No. 2002-296223
3. Problems to be Solved by the Invention
Insertion of the element 21 having the chamfers 28 formed on the rear end 27 prevents undesired deformation of the metallic terminal members 51 by virtue of the aforementioned reduction in insertion resistance of the element 21, or implementation of smooth insertion. However, the following additional problem has been found to arise. In some cases, the electrode terminals 25 of the element 21 to be inserted are damaged; for example, scraped or exfoliated. From tests and analysis of test results, the present inventors found the cause of such damage, as described below.
The electrode terminals are formed on the element as follows: a metallization paste which contains a high-melting-point metal, such as platinum, as a main component is applied by printing to the surface of a green element (ceramic), and then the applied metallization paste and the green element are simultaneously fired. The thickness of the electrode terminals is as thin as tens of μm at most. The rear ends of the electrode terminals 25 of the above-mentioned conventional element 21 are located at the same position as that of front ends 28a of the chamfers 28 (front ends of chamfered portions located on a side toward the front end of the element). In the course of inserting the element 21 having the chamfers 28 formed on the rear end 27, the element 21 slides, through relative movement, on the metallic terminal members 51 from the start of insertion, and a relatively large force which is generated until the metallic terminal members 51 pass over the chamfers 28, is applied directly to the rear ends of the electrode terminals 25 located at the front ends (outermost ends) 28a of the chamfers 28. Since the electrode terminals 25 are thin, and a relatively large force which is generated when the metallic terminal members 51 pass over the chamfers 28 is applied directly to the rear ends of the electrode terminals 25 located at the front ends 28a of the chamfers 28, the metallic terminal members 51 act directly on the corresponding electrode terminals 25 to scrape off the electrode terminals 25 in a frontward direction from their rear ends. The electrode terminals 25 are thus prone to damage. Damage, if any, to the electrode terminals in the terminal connection structure deteriorates the reliability of electrical connection when the sensor (product) is used in actual applications.